Steam-generating boilers, such as those typically used at electrical power generating plants, are generally large steel or masonry structures containing hundreds of vertically arranged steel tubes which are in thermal contact with a burning fuel, such as coal. The burning fuel heats water circulated through the tubes. The heated water or steam is used to drive turbines which generate electricity.
The tubes are generally organized in large panels or banks of parallel tubes which are connected together with a metal membrane or web continuously interposed between each pair of adjacent tubes in the bank to form a tube wall. The tubes generally have an outer diameter which can range from about 1 inch up to about 3 inches, with a wall thickness which can range up to about 0.5 inch. The web or membrane connecting adjacent tubes to each other generally has a thickness about equal to the wall thickness of the tubes, with the width of the webbing generally ranging from about 0.25 inch to about 0.75 inch. The webs or membranes can be welded to the outer walls of adjacent tubes to form the tube banks; however, the tube and connecting membranes can be, and preferably are, formed together in a single casting operation.
On account of deterioration due to corrosion and the like, the tubes occasionally require replacement. Ordinarily, repair of damaged or worn tubing involves cutting and removal of relatively large sections of the tube banks, and replacement with a new panel of tubes. The bank sections are generally replaced in large rectangular sections typically having sides ranging from a few feet to over 10 or even 20 feet in length.
The sections of tube banks which are to be replaced are generally cut out using a power saw or a cutting torch. A preferred method and apparatus for cutting out a section of boiler tube wall is illustrated and described in U.S. Pat. No. 5,033,347.
After the damaged or worn section of tube wall which is to be replaced has been cut out and removed, it is first necessary to properly prepare the tube ends exposed by removal of the old section before positioning the new tube bank section in the tube wall opening. Proper preparation of the exposed tube ends of the existing boiler tube wall requires chamfering or bevelling of the exposed tube ends to facilitate a good weld between the new tube wall section and the existing boiler tube wall. More specifically, the adjoining tube ends of the existing tube wall and those of the new or replacement section of tube wall should be provided with a frustoconical bevel so that when the adjoining tube ends are placed in alignment, a circumferential groove is formed to receive molten metal from the welding rod. The replacement section of tube wall is usually fabricated and prepared for welding off-site before the damaged or worn section of existing tube wall is removed. The exposed tube ends of the existing tube wall, however, must be prepared for welding on-site, often in a confined space. It is highly desirable that all work performed on the existing tube wall be conducted as quickly and efficiently as possible so as to minimize the period during which the steam generator is taken out of service for repair. Accordingly, preparation of the exposed tube ends of the existing tube wall for welding is generally accomplished using high-speed, portable, hand-held milling tools such as those disclosed in U.S. Pat. Nos. 4,449,871 and 4,889,454.
To facilitate rapid and efficient chamfering or bevelling of the exposed tube ends of the existing tube wall using conventional high-speed milling tools, and to provide ample space between vertically adjacent tubes for welding around the entire circumference of the tube ends, it is highly desirable and generally necessary to remove a portion of the membrane between each pair of exposed tube ends of the existing tube wall. Typically, it is desirable to remove the membrane, between adjacent tube ends which are exposed by removal of the damaged wall section, from the horizontally cut edge of the existing boiler tube wall to a depth which is at least equal to the depth of the frusto-conical bevel which is to be milled at the tube end. Typically, membrane material is removed from the horizontally cut edge between two adjacent tubes to a depth of from about 3/8 inch to about 3/4 inch from the cut edge. Heretofore, removal of the membrane material from the area between adjacent tube ends of the existing tube wall has been achieved using a cutting torch or portable power tools such as a grinding tool having a rotating abrasive disk or with a circular saw. None of the conventional methods of removing the membrane material from between tube ends is entirely satisfactory. Cutting torches are much faster and less prone to failure and damage than power tools, such as saws and rotating abrasive disks, but expose the tube walls to extremely high temperatures which can weaken or damage the tubes. The presently preferred method of removing membrane material from between the tubes is to make two vertical cuts, generally at the junctures between each of the two adjacent tubes and the intervening membrane, to form a tab which is folded or bent away from the area of the tube ends. When making the two vertical cuts along each side of the membrane, care must be taken to avoid removal of material which would reduce the nominal or effective thickness of the tube wall. After the pair of cuts have been made in the membrane, and the material between the cuts has been bent away from the tube ends, it is then generally necessary to grind the excess material from the tube to provide a uniform wall thickness around the circumference of the tube ends which are to be bevelled. These techniques are very time-consuming, generally requiring several minutes for each pair of tubes.